1. Field of the Invention
The present invention relates to an optical communication system, optical receiver and wavelength converter, capable of processing optical signals at different transmission speeds.
2. Description of the Related Art
In recent arterial optical communication systems, for example, there have been developed various optical modulators and electronic circuits adaptable to high transmission speeds in the systems aiming at long transmission distances and increased transmission capacities. However, if a transmission speed reaches a range of 10 Gb/s or higher, it becomes extremely difficult to realize electronic circuits and the like which operate at higher speeds. Therefore, there has been actively made research and development of systems for aiming at increased capacities such as by wavelength division multiplexing techniques. Under these circumstances, a large number of components such as optical senders and optical receivers have become installed onto optical communications apparatus, and also there has been proposed such a system to transmit optical signals at different transmission speeds through a single optical fiber.
There will be briefly explained hereinafter conventional optical receivers.
In a typical optical receiver, an optical signal transmitted via an optical transmission path is optical-electric converted by a light receiving element into an electrical data signal. The thus converted data signal is branched into two branched signals, one branched data signal being equalized and amplified, to be input to a decision circuit; and the other branched data signal being sent to a clock extracting section that obtains deciding timing. The clock extracting section is typically constituted of: an EXOR circuit or a combination of differential circuit and full-wave rectifying circuit; and a filter such as an SAW filter or a dielectric filter, so as to send a extracted clock signal to the above described decision circuit. Then, the one branched data signal is decided at the decision circuit in accordance with timing of the extracted clock signal.
As the clock extracting section mentioned above, there is frequently used a phase-lock loop (PLL). In this case, the one branched data signal branched in the same manner as the above is input to the decision circuit, and the other branched data signal is input to a phase comparator that compares a phase of the other branched data signal with a phase of an output of a voltage controlled oscillator (VCO) constituting the PLL circuit. Then, due to a loop constitution in which an output of the phase comparator is used as a controlling signal of the VCO, the synchronization of the VCO with the other branched data signal is made, so that the output of the VCO is output as the clock signal.
However, the above described conventional optical receiver has such a defect that the optical receiver can receive only an optical signal at a single transmission speed, because the frequency of clock signal capable to be extracted is fixed due to constraints such as a band of the filter and a controlling range of the VCO, to be used in the clock extracting section. Further, there may be caused constraints on the transmission speed of the receivable optical signal, also due to the band of the equalization and amplification to be performed at the data deciding side. Accordingly, in the above described case where optical signals at different transmission speeds are to be transmitted through a single optical fiber, it is required to provide a dedicated optical receiver for each transmission speed, thereby complicating the constitution of the optical communication system.
The present invention has been carried out in view of the above, and it is therefore an object of the present invention to provide an optical communication system and an optical receiver with a simplified constitution, capable of receiving and processing optical signals at different transmission speeds. It is a further object of the present invention to provide a wavelength converter adaptable to a plurality of transmission speeds, by mutually combining the optical receiver and optical.sender according to the present invention.
Note, the present applicant has proposed an optical receiver for receiving and processing optical signals at different transmission speeds within a single apparatus, in the prior patent application (see Japanese Patent Application No. 10-69694). However, the present invention differs from the invention of the prior patent application, in terms of detecting system of transmission speeds of received optical signals.
To this end, the present invention provides an optical communication system including an optical sender that transmits optical signals at different transmission speeds and an optical receiver that receives a transmission light from the optical sender to thereby perform a data decision processing, wherein the optical sender comprises superimposed signal generating means for generating a superimposed signal corresponding to a transmission speed of transmission light, to place the superimposed signal onto the transmission light, and the optical receiver comprises speed detecting means for detecting the transmission speed based on the superimposed signal included in a received light, and data processing means for performing a data decision processing of the received light according to the transmission speed detected by the speed detecting means.
In the optical communication system having such a constitution, the transmission light output from the optical sender includes the superimposed signal corresponding to the transmission speed thereof, while the transmission speed is detected at the optical receiver having received the transmission light, based on the superimposed signal included in the received light, so that the data decision processing is performed corresponding to the transmission speed of the received light. Thus, it becomes possible to receive and process optical signals at different transmission speeds by a single optical receiver, even when optical signals at different transmission speeds are transmitted between the optical sender and optical receiver, thereby enabling to provide an optical communication system of a simplified constitution.
The present invention further provides an optical receiver that receives optical signals at different transmission speeds to perform a data decision processing, wherein the optical receiver comprises speed detecting means for detecting a transmission speed of a received light, based on a superimposed signal which corresponds to the transmission speed and is included in the received light, and data processing means for performing a data decision processing of the received light, according to the transmission speed detected by the speed detecting means.
In the optical receiver having such a constitution, the transmission speed of the received light is detected based on the superimposed signal included in the received light, to thereby execute the data decision processing corresponding to the transmission speed of the received light. Thus, it becomes possible to process optical signals at different transmission speeds by a single optical receiver.
In the aforementioned optical receiver, the data processing means may comprises an optic-electric converting section that converts the received light into an electrical signal, an equalizing section that linearly amplifies an output signal from the optic-electric converting section, a clock extracting section that extracts a clock signal based on the output signal from the optic-electric converting section, making use of a phase-lock loop circuit with an operation setting thereof being switched corresponding to the transmission speed detected by the speed detecting means, and a decision processing section that performs a data decision processing of an output signal from the equalizing section, making use of the clock signal extracted by the clock extracting section.
According to such a constitution, the operation setting of the phase-lock loop circuit of the clock extracting section is switched to a setting suitable for the transmission speed detected by the speed detecting means, so that a clock signal can be extracted reliably even when the transmission speed of the received light is changed, thereby enabling to precisely receive and process optical signals at different transmission speeds by performing the data decision processing making use of the extracted clock signal.
As a specific constitution of the clock extracting section, the phase-lock loop circuit of the clock extracting section may include a plurality of controlled type oscillators corresponding to a plurality of transmission speeds, respectively, and any one of the plurality of controlled type oscillators may be driven according to the transmission speed detected by the speed detecting means. Alternatively, the phase-lock loop of the clock extracting section may include a single controlled type oscillator and a frequency converter that converts the frequency of an oscillation signal to be output from the controlled type oscillator, and a conversion ratio of the frequency converter may be set according to the transmission speed detected by the speed detecting means.
Further, for the data processing means, the equalizing section may have a variable equalizing band, which is controlled according to the transmission speed detected by the speed detecting means.
According to such a constitution, the operation setting of the clock extracting section and the equalizing band of the equalizing section are controlled at optimum corresponding to the transmission speed of the received light, thereby enabling to precisely receive and process optical signals at different transmission speeds.
The present invention further provides a wavelength converter that converts an optical signal input thereto, into an optical signal at the frequency different from the frequency of the input optical signal, wherein the wavelength converter comprises speed detecting means for detecting a transmission speed of input light, based on a superimposed signal which corresponds to the transmission speed and is included in the input light, data processing means for performing a data decision processing of the input light, according to the transmission speed detected by the speed detecting means, output light generating means for generating an output light having a wavelength different from the wavelength of the input light and modulated in accordance with a data signal indicative of a processing result by the data processing means, and superimposed signal generating means for generating a superimposed signal corresponding to the transmission speed detected by the speed detecting means, to place the superimposed signal onto the output light.
The wavelength converter having such a constitution can be regarded as being provided by combining the optical sender to an output stage of the optical receiver that is used in the optical communication system, thereby enabling to realize a wavelength conversion even for input lights at different transmission speeds.